The CAS (Check and Set) operation is a way to handle concurrent updates in Memcached. It provides a mechanism to ensure that the value of a key hasn't changed before updating it. This is useful when multiple threads or processes may be updating the same key, to avoid lost updates.
Let's look at a few examples in Java using the "spymemcached" client library:
MemcachedClient c=new MemcachedClient(new InetSocketAddress("localhost", 11211)); // Perform some operation c.set("key", 3600, "original value"); CASValue casValue = c.gets("key"); System.out.println("CAS Value: " + casValue);
In this example, we first set a key, then retrieve it along with its CAS value using the gets
method. The CAS value can be used later to perform a CAS update.
long casId = casValue.getCas(); // Perform a CAS update CASResponse casRes = c.cas("key", casId, "new value"); if (casRes.equals(CASResponse.OK)) { System.out.println("CAS update successful."); } else { System.out.println("CAS update failed."); }
Here we use the cas
method to attempt a CAS update. If the value has not been modified by another process since we retrieved it, the update will succeed. Otherwise, it will fail.
1. Can CAS entirely prevent lost updates? Not completely. If two processes retrieve the same CAS value for a key, and then both try to update it, one will succeed and the other will fail. The second process will need to handle this failure, possibly by retrying the operation.
2. What happens if the key being updated with CAS doesn't exist? If you attempt a CAS operation on a key that doesn't exist, Memcached treats it as an add operation and adds the key-value pair.
Dragonfly is fully compatible with the Redis ecosystem and requires no code changes to implement.